While the following exposition of the invention will refer mainly to the important application of water desalination and purification by reverse osmosis, it will be understood that the invention encompasses application of the same principles to any other fluid separation process in which applied pressure is used to drive one component of a fluid mixture across selectively permeable membranes.
Fluid separation by a pressure-driven membrane process is achieved by pumping the feed stream (in reverse osmosis, typically saline water) to a suitably elevated working pressure, and conveying the pressurized feed stream into a pressure vessel containing a suitable array of semi-permeable membranes. The feed stream is circulated through feed channels on the high pressure side of the membranes, with a suitable flow velocity to alleviate adverse boundary layer effects by forced convection. A permeate fraction (in reverse osmosis, product water of greatly reduced salinity) will permeate across the membranes into low pressure collection channels if the working pressure sufficiently exceeds the feed stream osmotic pressure, with a considerable excess pressure required to achieve high flux and purity of the permeate.
While the purified permeate fraction of the feed is withdrawn from the low pressure collection channels of the membranes, the feed stream becomes concentrated along the feed channels, and a rejected concentrate fraction must be withdrawn from the high pressure side of the membranes in order to prevent excessive salt concentrations over the membranes. The concentrate stream may be a large fraction of the feed stream; and in the case of sea water reverse osmosis desalination the concentrate stream is typically 70% to 80% of the feed stream. The concentrate stream is removed from the membranes at nearly the full working pressure, and thus carries a large fraction of the pressure energy that was imparted to the feed stream.
It must be emphasized that the working conditions of the membranes in a given installation must be maintained within certain design limits for satisfactory performance and life. Thus, the working pressure, feed channel flow velocity, concentrate flow, and feed/concentrate ratio must each be maintained between upper and lower limits. Accordingly, the external hydraulic devices supplying the feed stream and removing the concentrate stream must be correctly matched to the membranes under all conditions.
However, actual working conditions are sensitive to the feed conditions (salinity and temperature) which may be widely variable as in the case of shipboard desalination systems; and to the condition of the membranes (fouling and flux decline) which will certainly be variable over the life of the installation. Since the working conditions are always drifting at least somewhat unpredictably because of membrane of feed changes, the hydraulic devices on feed and concentrate streams must adapt to restore correct hydraulic matching to the membranes. In prior art reverse osmosis systems, these hydraulic devices have been unable to provide fully self-regulating adaptation under a full range of changing conditions without operator intervention, while also approaching highest energy efficiency.
Before the concentrate stream is exhausted, it must be depressurized by an appropriate hydraulic device. In most prior art reverse osmosis installations, the concentrate stream is depressurized by throttling over a back-pressure valve, with the characteristics of either an adjustable orifice or relief valve. The back-pressure valve provides means to regulate the membrane working pressure by suitable adjustment, but dissipates all of the pressure energy carried by the concentrate stream. Such reverse osmosis installations are highly inefficient, may have difficulties with erosion or plugging of the orifice, and often require operator intervention to adjust the back-pressure valve for proper matching to feed flow and membrane operating conditions.
In some prior art reverse osmosis systems, the concentrate stream is depressurized by expansion through an energy recovery device. Reciprocating energy recovery pumps have been found to be practicable for relatively small reverse osmosis systems. A class of reciprocating energy recovery pump operates with a strictly constant ratio between feed and concentrate streams, determined by the fixed ratio of displacements between pumping and expansion chambers. With a fixed feed/concentrate ratio, permeate flow is directly proportional to pump speed, and working pressure is determined entirely by membrane resistance to the imposed permeate flow. Such energy recovery pumps may be operated at variable speed to provide favourable matching with solar or other unsteady power sources as discussed by Keefer et al (B. G. Keefer, R. D. Hembree and F. C. Schrack, "Optimized Matching of Solar Photovoltaic Power with Reverse Osmosis Desalination", Desalination 54, 89-103, (1985)). However, a fixed feed/concentrate ratio can be disadvantageous when the pump is operated at constant speed, and feed water conditions (salinity, temperature) vary widely. Thus in shipboard installations with constant permeate flow, working pressure will vary too widely with the normal variation of sea water salinity and temperature between arctic and equatorial oceans. A selfregulating characteristic with a declining ratio of permeate to feed flow at higher pressure would be preferable, so that working pressure would vary less widely with changing membrane flux.
It is also well known in the prior art to use a hydraulic turbine for energy recovery from the concentrate stream. Both impulse turbines such as Pelton turbines, and reaction turbines such as radial turbines or reverse-running centrifugal pumps, have been used in reverse osmosis systems, with the turbine usually coupled as an auxiliary power source to the high pressure pump pressurizing the feed stream to the working pressure. Application and control of Pelton turbines is discussed by Woodcock et al (D. J. Woodcock and I. M. White, Desalination 39, 447 (1981)). Use of reverse-running centrifugal pumps as energy recovery turbines for reverse osmosis is discussed by Raja et al (W. A. Raja and R. W. Piazza, Desalination 38, 123, (1981)).
When the feed pump is a centrifugal pump, the energy recovery turbine may conveniently operate at the same shaft speed and may be coupled directly to the feed pump. According to the selection of prime mover, the pump shaft speed is usually substantially constant. Because of variations in the feed conditions and permeability of the membranes, turbine entry pressure and flow of the concentrate stream will vary. Thus, an adjustable nozzle will be required for a Pelton turbine (as described in the cited reference by Woodcock et al), and will be desirable for a constant shaft speed radial turbine. An adjustable throttle valve is frequently also provided between the feed pump and the membranes, or between the membranes and the energy recovery turbine, to compensate for mismatch of hydraulic conditions between the pump, turbine and membranes. Use of throttle valves to correct hydraulic matching errors is of course wasteful of energy, and complicates system operation and control. Finally, it is common practice to provide a by-pass valve (in parallel with the energy recovery turbine, as described in the cited reference by Raja et al) for the concentrate stream, so that the energy recovery turbine can be hydraulically engaged after the reverse osmosis system has been started. This means that the pump drive motor must be over-sized, and again implies relatively complicated operating and starting procedures. It is seen that prior art use of conventional hydraulic turbines for reverse osmosis energy recovery has entailed a multiplicity of auxiliary control elements, including valves and/or nozzles, for starting and operating adjustments.
Well-known prior art energy recovery pump configurations use a multistage centrifugal pump connected directly to a radial energy recovery turbine, and to a constant speed electric motor prime mover. As noted above, such configurations have required auxiliary valves and relatively complicated operating procedures, to compensate for inevitable maladjustments between the membrane operating conditions and the optimum hydraulic operating points of pump and turbine. Because the head/flow characteristics or centrifugal pumps and radial turbines operating at constant shaft speed have moderate slopes near their optimum operating points, a small drift in membrane operating conditions would cause a relatively large change in permeate flow and large departures from optimal pump and turbine hydraulic conditions, unless a throttle valve is provided as a means of adjusting pressure at a given flow. The throttle valve is thus normally provided as a manual means of adjustment to maintain membrane productivity against flux decline, while also keeping pump and turbine conditions approximately constant. Changes in membrane operating conditions may alternatively be compensated by using a variable speed drive for the pump-turbine combination, as proposed by Fechner et al. (G. Fechner and R. Pillkahn, Desalination 55, 461, (1985)). In applications where electric motors are the prime movers, a variable speed drive will entail substantial additional cost and some efficiency losses.
A related prior art invention by Kohler (U.S. Pat. No. 4,321,137) shows the stages of the feed pump separated into first and second pumps in series. The energy recovery turbine is coupled directly to the first (lower pressure) feed pump, although Kohler points out that it may be coupled to either pump. Both pumps are driven through clutches by electric motors. Once the system has been started and the turbine has been hydraulically engaged, the motor connected to the first pump is disengaged because the turbine drives this pump. In keeping with conventional practice as discussed above, Kohler provides a throttle valve to provide pressure adjustments between the feed pumps and the membranes, and a bypass valve in parallel with the energy recovery turbine. Kohler also provides a controllable speed motor for the second pump, as a further means of optimizing hydraulic matching. This invention illustrates that the turbine can be the running power source to a section of the feed pump which can then be disengaged from its starting motor, but does not provide any simplifications of apparatus or operating procedure. A self-regulating reverse osmosis apparatus is not described, as the turbine and first pump are brought up to a pre-determined shaft speed by their starting motor, and other auxiliary control means (throttle valves and a speed control on the second pump) are provided for system regulation.
The present invention provides a free rotor booster pump to accomplish the final pressurization of the feed stream before entering the membrane feed channels. The free rotor booster pump includes a pump and an energy recovery turbine powered by the concentrate stream, with the turbine as the sole power source to the pump. No external motor or other power source is required to drive or start the free rotor booster pump. The free rotor booster pump is self-starting (once a flow of feed fluid has been initiated) and provides by passive adjustment of its own shaft speed a fully self-regulating control to the reverse osmosis apparatus, with the highly desirable characteristic that the ratio of permeate flow to feed flow declines at higher working pressures. No auxiliary throttle valves or bypass valves are required for starting or running regulation, and the only operator control required is to turn the feed stream supply means on or off.
The use of a turbine powered by one fluid streams as the direct and only prime mover to a pump pressurizing a second fluid stream is of course well known in diverse prior art applications other than reverse osmosis. Free rotor centrifugal compressors driven by turbines are well known in twin spool gas turbines and automotive turbochargers. Schwartzman (U.S. Pat. No. 4,067,665) describes a free rotor turbine-driven centrifugal booster pump used as a pressure intensifier, so that a portion of the feed stream at an initial pressure is used to power the turbine and the remainder of the same feed stream is pressurized by the pump to a higher delivery pressure. Likewise, it is well known to use the free rotor combination of a positive displacement rotary pump (such as a vane or gear pump) and a similar positive displacement hydrostatic motor, for diverse applications such as fluid pressure intensifiers and proportioning flow dividers.
While free rotor turbine-driven pumps and compressors have an extensive prior art in other applications, the present invention obtains substantial and unexpected benefits from the use of a free rotor booster pump for final pressurization of the feed fluid to a pressure-driven membrane separation process. In addition to recovering energy from the reject stream to assist the feed pressurization, the free rotor booster pump serves as a fully self-regulating control element for the reverse osmosis or similar membrane separation apparatus. Passive speed adjustment of by the free rotor booster pump enables it to accommodate changing feed and membrane conditions over a wide range of conditions, maintaining good hydraulic matching between pump and turbine elements and the membranes. Other auxiliary control elements (such as throttle valves, adjustable nozzles, and variable speed pump drives) can be eliminated, for great simplification of both apparatus and process.
In order to appreciate the importance of the present invention, it must be realized that the fraction of the feed stream permeating the membrane is highly sensitive not only to applied pressure, but also to feed stream concentration and temperature, and the fouling and aging history of the membrane. The matching problem is compounded by the fact that centrifugal pumps and turbines have severely degraded performance when operated off their design flow and pressure parameters, while the changing hydraulic conditions of the membranes make the pressure and flows indeterminate. The free rotor booster pump allows its turbine and pump elements to seek their own most efficient combined operating point for any conditions of the feed and membranes, while also providing the desirable self-regulation control characteristic of reduced permeate/feed flow ratio for increased pressure. Operation is self-starting and stable upon initiation of feed stream flow. Operating complexity, equipment complexity, and energy losses of throttle valves and variable speed transmissions are avoided.